Recently, products, such as paper handphones, flexible displays, and flexible batteries, have been developed as a technique of integrating a flexible device on a flexible substrate has advanced. However, more technological development may be required for real industrialization of the above products. The biggest limitation for the industrialization is to ensure reliability when mechanical deformation, such as bending, tension, compression, and torsion, is applied during the operation of a flexible device. With respect to an actual electronic device, a metal electrode has been more frequently used in real industry due to high electrical conductivity, low cost, and ease of processing in comparison to a material such as graphene and conductive oxide. Recently, in order to increase a degree of mechanical deformation of metal electrodes and wirings, wiring structures in a wavy shape, an arc shape, and a horseshoe shape have been suggested. These structures may increase the degree of deformation by using an extra deformation structure that decreases local instability of the metal electrode. These structures may allow a deformation of 50% or more. However, an actual fracture of a flexible device may occur under deformation conditions in which a strain is lower than a fracture strain but it is repetitive. Stability of the metal electrode with respect to fatigue facture is a very important issue. Actually, with respect to a bendable or foldable mobile phone, a cyclic deformation of 1 million times or more must be ensured. Thus, the stability of the metal electrode in a flexible device must be resolved for mass production.
Fatigue fracture is a fracture mode in which a facture of a material occurs under a condition of a repetitive load. The fatigue facture behavior of a metal thin film may be broadly divided into two categories, i.e., crack formation and crack propagation. The crack formation is related to the movement of dislocations during repetitive deformation. Since the movement of the dislocations may form protrusions such as extrusions or intrusions, stress may be locally concentrated. Eventually, crack formation may occur. After the formation of cracks, propagation of cracks, which increase the resistance of the metal electrode, may follow. Therefore, a technique capable of controlling the formation and propagation of cracks may be needed. However, research into a method of improving electrical and mechanical reliabilities of the metal electrode under high cycle conditions is insignificant.